1. Field of the Invention
The present invention relates to silicon nitride sintered bodies which have superior mechanical properties at room temperature, with a minimized scattering of the properties, and also good productivity and cost efficiency.
2. Description of the Prior Art
Silicon nitride is a material well balanced in strength, fracture toughness, corrosion resistance, wear resistance, thermal shock resistance and oxidation resistance, etc., and has been extensively used in a wide variety of applications, such as cutting tools, frictionally sliding parts or other structural materials. However, there is the problem that this ceramic material is poor in strength and reliability as compared with metallic materials.
One reason for the poor strength of the silicon nitride sintered body is closely related to the grain boundary phase in the silicon nitride sintered body. The grain boundary phase is composed of a glass phase formed from a sintering aid, which is an indispensable component for sintering silicon nitride. In general, since this glass phase is brittle as compared with the matrix phase of the sintered body, it is highly susceptible to breakage when stress concentration is applied to the grain boundary phase. This causes the lowering of the strength of the sintered silicon nitride sintered body.
Therefore, various methods have hitherto been tried to improve the strength by reducing the grain boundary phase of a silicon nitride sintered body. For example, Japanese Patent Application Laid-Open (Kokai) Nos. 2-70715 and 3-117315 disclose techniques for reducing the thickness of a grain boundary phase by forming a grain-refined structure composed of equi-axed crystal grains of .alpha.-Si.sub.3 N.sub.4 and prismatic crystal grains of .beta.-Si.sub.3 N.sub.4. However, in order to obtain fine .alpha.-type crystal grains, a fine Si.sub.3 N.sub.4 powder having a high .alpha.-ratio should be used as a starting material powder, resulting in a high production cost. Further, in order to ensure an improved strength properties in the resultant silicon nitride sintered body, the conversion rate to .beta.-crystallization should be increased by sintering. However, in this sintering process, the .beta.-type crystal grains grow to 2 .mu.m or more. Therefore, there is limitation in reducing the grain boundary phase only by the above-mentioned structural refinement technique.
Further, as disclosed in Japanese Patent Laid-Open Nos. 61-91065 and 2-44066, there has been known a method for combining equi-axed .alpha.'-sialon having the general formula Mx(Si,Al).sub.12 (O,N).sub.16 wherein M is at least one member selected from the group consisting of Mg, Ca, Li and rare earth elements and prismatic .beta.'-sialon. This method improves the mechanical properties, such as strength, by the formation of a composite crystal phase. However, as is also apparent from the working examples disclosed in these applications, all the sintered bodies that stably have a bending strength exceeding 100 kg/mm.sup.2 are obtained by a hot pressing process and this method is inappropriate to stably ensure high strength properties on an industrial scale.
As a further attempt for achieving an improved strength, fine foreign particles are dispersed in the structure of a silicon nitride sintered body to provide a composite structure. For example, Japanese Patent Laid Open No. 4-202059 discloses a method in which fine particles having a size of 1 to 500 nm are dispersed in prismatic silicon nitride or sialon having an average minor axis of 0.05 to 3 .mu.m and an aspect ratio of 3 to 20. However, although the working examples shows 167 kg/mm.sup.2 as the highest strength, this method is highly liable to bring about the deterioration of the strength due to the presence of coarse silicon nitride and the Weibull coefficient is about 9 at the highest level. Therefore, there is a problem in stably obtaining high strength properties.
Japanese Patent Laid-Open No. 4-295056 discloses a method for dispersing particles of a foreign substance in the grain boundary phase of prismatic silicon nitride crystal grains. However, in this method, the prismatic silicon nitride crystal grains have a minor axis diameter and major axis diameter reaching, at the most, 2 to 3.5 .mu.m and 10 to 14 .mu.m, respectively. Therefore, the matrix per se acts as fracture origins and the strength shown in the working examples is only 158 kg/mm.sup.2 at the highest. Further, since a high firing temperature of 1800.degree. C. or higher is required, the method is unsatisfactory also in productivity and cost.